Extremely precise control and regulation of analogized digital valves in modern vehicle brake systems with hydraulic pressure regulation constitutes an increasingly important prerequisite for a multiplicity of different regulation functions and comfort functions, such as for example systems for inter-vehicle distance regulation and speed regulation, such as are becoming ever more widely used. Through the use of pressure sensors in each individual wheel brake cylinder, it is possible at all times to perform a precise pressure measurement and, associated therewith, precise adjustment to the required brake pressure. This however leads to high cost outlay for the additional pressure sensors and, in association therewith, higher overall costs for the brake system, which in turn has an adverse effect on the commercial competitiveness of such systems.
One possibility, known in the prior art, for avoiding the use of additional pressure sensors and associated additional costs can be achieved by gauging the opening and closing current in the form of a characteristic curve which correlates a pressure difference prevailing across the valve with an exciter current. This permits substantially precise regulation of an analogized hydraulic valve even without additional pressure sensors. Such a method is disclosed for example in DE 102 24 059 A1, which is incorporated by reference. In said method, the characteristic curve is stored electronically in the regulation system and, by means of the exciter current, it is subsequently possible to set a pressure difference in a targeted manner without having to resort to physically measured pressure data.
DE 10 2005 051 436 A1, which is incorporated by reference, likewise proposes a method for pressure regulation in a hydraulic brake system without the use of additional pressure sensors. Here, the analog-regulation hydraulic valves are calibrated by means of the ABS wheel rotational speed sensors provided on the vehicle, which determine a rotational speed decrease and thus a braking action as a function of the exciter current. This method permits valve calibration without the use of additional measurement sensors directly on the vehicle itself. The actuation characteristic curve determined in this way is electronically stored and used for the pressure regulation.
DE 10 2008 060 622 A1, which is incorporated by reference, discloses a hydraulic motor vehicle brake system which comprises a continuous-delivery pressure medium pump. Adjustment to a certain pressure demand can be performed by means of overflow regulation of the cut-off valve or of the outlet valve. Here, since the pressure adjustment is substantially dependent on the overflow behavior of the regulating valves and the actuation characteristic curve alone does not provide information regarding the flow passing through a valve, additional pressure sensors are used in the region of the wheel brakes.
The disadvantage of the methods known from the prior art for calibration and subsequent pressure regulation without additional pressure sensors consists in that the pressure regulation is performed taking into consideration only a single valve characteristic, specifically the actuation characteristic curve. By contrast, the actual throughflow or overflow behavior and the associated rate of change of pressure are not taken into consideration. In the prior art, if the overflow behavior is of particular significance, the information required for regulation is obtained by means of additional pressure sensors. Furthermore, each valve—like the pumps and pump motors used for the build-up of pressure—are subject to certain manufacturing tolerances owing to the production process; it is also the case that a calibration of said devices is possible only with limited accuracy. In the worst case, the various tolerances of the devices involved in the pressure regulation may add up in the same direction, with the result that, in the event of a pressure selection, severe and enduring pressure setting inaccuracies may arise in the system. This leads to regulation oscillations which are acoustically and haptically perceptible to the driver and which are often regarded as vehicle malfunctions.